1. Field of the Invention
The present invention relates to a measuring method and apparatus for measuring a non-linear parameter and/or its distribution in an acoustic medium. It also relates to the application of the method to measurements such as temperature measurement of human tissue for an ultrasonic diagnosis, hyperthermia, etc. The measuring apparatus of the present invention especially relates to a device for measuring a non-linear parameter of an acoustic medium such as biomedical tissue, measuring the space distribution of the non-linear parameter of the acoustic medium and displaying an image of the space distribution on a display unit. The measuring method and apparatus of the present invention is useful in many applications and primarily intends to provide special apparatus to measure the temperature of internal tissue noninvasively.
2. Discussion of the Prior Art
Usually, an ultrasonic apparatus such as an ultrasonic tomograph, is designed while assuming a pressure independent sound speed. In the first approximation, sound speed is regarded as a pressure independent constant and the density of the acoustic medium is also regarded as pressure independent for a liquid and a solid medium like tissue. However, more accurately, sound speed relates to the density (it is called dynamic density) at each infinitesimal portion of the acoustic medium through which the sound wave propagates, and because a sound wave is a compressional wave, the density of each part of the acoustic medium is varied by an increase in sound pressure.
The relationship between sound speed and density of a medium through which the sound wave propagates can be expressed as ##EQU1## This results in a nonlinearity relationship between sound speed and pressure expressed as ##EQU2## where, .DELTA..rho.: dynamic density change caused by .DELTA.p;
.DELTA.P: dynamic sound pressure, that is a variation of the pressure caused by the sound wave propagating in the acoustic medium; PA1 C: sound speed resulting under dynamic sound pressure .DELTA.p; PA1 C.sub.0 : sound speed (phase velocity) resulting under static pressure, that is sound speed of infinitesimaly small amplitude propagating through the medium; PA1 .rho..sub.0 : static density, density of the acoustic medium under static pressure; and PA1 B/A: a non-linear parameter of the acoustic medium. PA1 P is sound pressure of the pumping wave; and PA1 C, C.sub.0, and .rho..sub.0 are the same as defined in equation (1), respectively.
Equation (1) is applicable only under a second order approximation where the higher order terms have been neglected or eliminated from the equation. If the second order term is neglected, equation (1) keeps only first order term resulting linear acoustic equation; that is, sound speed is pressure independent as mentioned above as a "first approximation". More details about the non-linear acoustics are explained, for example, by T. G. MUIR, E. L. CARSTENSEN in 1979 (Prediction of Nonlinear Acoustic Effects at Biomedical Frequencies and Intensities; Ultrasound in Med. & Biol., Vol.6; Pergamon Press Ltd.).
Recently, the non-linear parameter B/A has become worthy of notice in the field of ultrasonic technology, because it includes novel information such as: the structure, the coupling state, the visco-elasticity, and the temperature of biomedical tissue; the chemical state of a body fluid; the metabolic activity; and so on. However, the quantity B/A is so small that it has been difficult to put it into practical use. To overcome this difficulty, an equivalent non-linear parameter (B/A).sub.e was introduced by N. Ichida, T. Sato, O. Ikeda and M. Linzer (Ultrasonic Imaging of the Non-linear Parameter of Tissue Using Scanned Low Frequency Pumping Waves and High Frequency Probe Waves) at the 7th International Symposium On Ultrasonic Imaging And Tissue Characterization, held on Jan. 6-9, 1982, sponsored by NBS, and in Japanese Patent Application No. TOKUGANSHO 57-167036, in 1982 (Ultrasonic Diagnostic Processing System) by the inventors. This new measuring method introduced a second sound wave (pumping wave) into the acoustic medium to be measured, to detect the second order term in equation (1). Therefore, two kinds of sound waves are superimposed in the acoustic medium. One is a "probing wave" to measure the non-linear parameter of the medium, and the other is "pumping wave" to generate the .DELTA.p. The amplitude of the pumping wave is made sufficiently higher than the probing wave, so that the non-linear parameter is enhanced to make it easy to measure. The non-linear parameter measured in this way is called "equivalent non-linear parameter (B/A).sub.e ", and equation (1) is modified as follows ##EQU3## where, (B/A).sub.e is an equivalent non-linear parameter;
At first a measuring method of the parameter (B/A).sub.e by the prior Patent Application will be described briefly. A pumping wave consists of a continuous wave (CW), whose wave length is varied. A probing wave is also CW and is applied to the medium by a pair of ultrasonic transducers; one of which transmits the probing wave and the other receives the probing wave. The phase of the transmitted probing wave is modulated by the pumping wave as it crosses the probing wave beam through the medium. The parameter (B/A).sub.e along the medium can be obtained by a Fourier transformation of the phase deviations of the received probing wave caused by the pumping wave corresponding to the various wavelengths of the pumping wave.
The above-mentioned prior art method of measuring the parameter (B/A).sub.e, however, had the following problems in attaining a good result rapidly.
(1) The frequency of the pumping wave had to be varied very widely, however, it is not normal for a transducer to have the necessary frequency band-width capable of transmitting the pumping wave effectively.
(2) The pumping wave had to be varied though many frequencies to get a high degree of resolution, so the apparatus received plenty of time to accumulate the data, and to perform the Fourier transformation.